This invention relates to the production of sealed electrochemical storage cells, and, more particularly, to the glass sealing of such cells.
Rechargeable cells are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as useful power. A number of such cells are usually connected together to form a battery having specific voltage or current delivery capability. Each cell includes an anode, a cathode, and an electrolyte separating the anode and the cathode. The most familiar types of electrochemical cells utilize a solid anode and a solid cathode, with a liquid electrolyte.
In another type of electrochemical cell, the anode and cathode are liquids, and the electrolyte is a solid ceramic material. One such electrochemical cell is the sodium-sulfur cell, which has a sodium anode, a sulfur cathode, and a solid electrolyte of beta double prime (Beta") aluminum oxide (alumina). Beta double prime aluminum oxide is formed from aluminum oxide, sodium oxide, and sometimes other oxides such as lithium oxide and magnesium oxide. The presence of the sodium permits sodium ion charge carriers to diffuse through the solid electrolyte at elevated temperatures during battery charging and discharging cycles.
Sodium-sulfur electrochemical storage cells can store several times as much charge per unit weight of cell than other high-performance rechargeable cells such as nickel-hydrogen cells. Sodium-sulfur electrochemical storage cells are therefore candidates for energy storage in both terrestrial and space applications. On earth, such storage cells may be used in electrically powered automobiles. In space, the sodium-sulfur cell may be used in satellites and other types of spacecraft.
The most common construction for a sodium-sulfur electrochemical storage cell is a tubular configuration. A tube of the Beta" electrolyte material is mounted within a larger cylindrical outer housing. Sodium is placed within the tube of electrolyte material, and sulfur is placed in the annular space between the electrolyte tube and the outer housing. Electrical connections are made at the ends of the cell, and the cell is heated to about 350 C. in service.
More recently, Beta" alumina flat-plate sodium-sulfur cells have been developed. The flat-plate cells utilize a flat piece of the Beta" electrolyte material separating a sodium anode and a sulfur cathode. The flat-plate cells have the advantage that the active area of electrolyte per unit weight of cell is larger than for corresponding tubular cells. Such flat-plate cells are discussed in U.S. Pat. Nos. 3,765,945, 3,783,024, and 5,053,294, for example.
One of the challenges in the fabrication of flat-plate cells is achieving dependable, long-life seals of the various components to each other. The cell must not leak molten sodium or sulfur out of their respective compartments, or permit external contaminants to intrude into the cell during operation at a temperature that is ordinarily about 350 C. Moreover, the cell is subjected to a range of temperatures during its operating life, and the thermal cycling between different temperatures can lead to failure of the seals as a result of the difference in the coefficients of thermal expansion, corrosion resistance, and mechanical properties of the components facing each other at the seal. Metals can be readily joined to each other as by welding, but it is often difficult to reliably seal ceramics to other ceramics, or ceramics to metals, because of the differences in properties.
For example, in FIG. 3 of U.S. Pat. No. 5,053,294, the ceramic/ceramic seal of the ceramic electrolyte to an internal shoulder of the ceramic container is indicated at numeral 18. At another location, metal weld rings are used to attach the end plates to the remainder of the cell. The metal weld rings must be attached to the ceramic container with a metal/ceramic seal, indicated at numeral 42 of the '294 patent.
Operable sealing structures and methods are disclosed in the '294 patent. Nevertheless, there is always a need for improved sealing procedures that are less complex, more reliable, and result in seals that are less subject to failure during the extended service life of the storage cell. The present invention fulfills this need, and further provides related advantages.